Abstract

The Upper Jurassic Sundance Formation in Wyoming has revealed unequivocal evidence for the operation of diurnal tides in the Western Interior Basin in the form of neap/spring bundles of foresets deposited by migrating subtidal sand waves. The upper part of the Sundance Formation comprises a thin, transgressive coquina, a mudstone-siltstone unit with a slight coarsening upward trend (40 m), and a sandstone unit with an erosive base (20 m). The mudstone-siltstone unit is marine, intensely bioturbated and includes in its upper levels numerous thin, discrete beds of wave-ripple laminated sandstone and storm-wave generated coquinas. The sandstone unit with an erosive base exhibits planar and trough cross-bedding, current ripple lamination and flat lamination with parting lineation, and is considered to have been deposited by tidal currents due to: 1. a bimodal, bipolar paleocurrent pattern; and 2. an abundance of reactivation surfaces and millimetre-scale mud drapes in sets of cross-bedding.

Occasionally, cross-bed foresets occur in bundles defined by distinct surfaces with the following sequence: reactivation surface→opposed flow current ripples → mud drape; or lower mud drape→opposed flow current ripples-Hipper mud drape. The foresets were deposited by the dominant tide, the current ripples by the subordinate tide and the clay drape(s) in stillstand periods between tides, implying extreme time-velocity asymmetry of the tidal currents. Within cross-bed sets the thickness of the foresets bundles varies gradationally between groups of thin (2 to 25 cm) and thick (40 to 110 cm) bundles with a pronounced periodicity of 9 to 14. These systemsatic variations in bundle thickness suggest neap/spring cycles in a diurnal tidal system with sand omission at low neap tides causing the periodicity to be slightly less than 14 on some occasions. These observations confirm the operation of tides in the basin and permit the nature of the tides to be interpreted in detail. In view of the mudstone-siltstone unit being wave-dominated and the sandstone unit with an erosive base being tide-dominated, the succession is interpreted as being a prograding barrier island-tidal inlet system with the mudstone-siltstone unit representing the lower beach face and the sandstone unit the tidal inlet, thus contradicting the offshore bar model of Brenner and Davies (1973, 1974).